Freeze-thaw aged polyethylene and polypropylene microplastics alter enzyme activity and microbial community composition in soil

In cold regions, microplastics (MPs) in the soil undergo freeze-thaw (FT) aging process. Little is known about how FT aged MPs influence soil physico-chemical properties and microbial communities. Here, two environmentally relevant concentrations (50 and 500 mg/kg) of 50 and 500 µm polyethylene (PE) and polypropylene (PP) MPs treated soils were subjected to 45-day FT cycles (FTCs). Results showed that MPs experienced surface morphology, hydrophobicity and crystallinity alterations after FTCs. After 45-day FTCs, the soil urease (SUE) activity in control (MPs-free group that underwent FTCs) was 33.49 U/g. SUE activity in 50 µm PE group was reduced by 19.66%, while increased by 21.16% and 37.73% in 500 µm PE and PP groups compared to control. The highest Shannon index was found in 50 µm PP-MPs group at 50 mg/kg, 2.26% higher than control (7.09). Compared to control (average weighted degree=8.024), all aged MPs increased the complexity of network (0.19–1.43%). Bacterial biomarkers of aged PP-MPs were associated with pollutant degradation. Aged PP-MPs affected genetic information, cellular processes, and disrupted the biosynthesis of metabolites. This study provides new insights into the potential hazards of MPs after FTCs on soil ecosystem in cold regions. In cold regions, microplastics (MPs) in the soil undergo freeze-thaw (FT) aging process. This study shows that MPs experience aging in natural soil after freeze-thaw cycles (FTCs), resulting in alterations to soil enzyme activity, microbial community structure, disruption of bacterial community co-occurrence networks, and inhibition of metabolic pathways. Notably, global climate change often leads to more frequent FTCs in natural environments. FT aging of MPs in natural soils at mid- to high-latitudes, where FTCs are more frequent, may increase their potential risk to soil ecosystems and should draw more public attention.